Optically- and electrically-addressable concentrators of biological and chemical materials

ABSTRACT

A concentrator for detecting biological and/or chemical materials in an environment. The concentrator comprises an engineered superlattice structure having alternating layers of elemental, binary or ternary group III-group V, or group IV-group IV semiconducting materials. A method for detecting biological and/or chemical materials in an environment using the concentrator. The method comprising exposing the concentrator to the biological and/or chemical materials in an environment and activating the superlattice structure optically or electrically followed by the detection of the biological and/or chemical materials.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) to co-pendingU.S. Patent Application No. 60/316,655 (filed on Aug. 30, 2001) entitled“OPTICALLY-ADDRESSABLE CONCENTRATORS OF BIOLOGICAL AND CHEMICALMATERIALS,” the contents of which are hereby expressly incorporatedherein in their entirety by this reference.

I. BACKGROUND

1. Field

This invention relates to the field of compact devices for remotelydetecting the presence of biological and/or chemical materials using thereversible binding of specific molecules onto a superlatticeconcentrator structure in order to quickly manipulate and identify thesemolecules.

More particularly, it pertains to the use of a new class of solid-stateconcentrators of biological and/or chemical agents made from engineeredGroup III-V semiconductor superlattice structures, surface bindingproperties of which can be controlled and manipulated by opticalillumination or other means, such as electrical biasing.

2. Description of the Related Art

Early detection and identification of the presence of biological agentsis important because it would allow initiation of rapid evacuation of anaffected area, start of immediate decontamination procedures, andprovide needed information for medical treatment of personnel.

There exist a large number of known detection methods used to identifychemical and biological materials. As discussed below, all currentlyused methods are limited in terms of sensitivity, response time,possibility of miniaturization, or a combination thereof. The mostcommonly used methods are as follows:

(a) Gas and Liquid Chromatography. According to these analyticaltechniques, an analyzed blend is separated based on the Nernstiandistribution of the components of the blend between two phases.

In the case of gas chromatography, the blend is first evaporated andthen directed to a solid adsorbent where the blend is separated. In thecase of liquid chromatography, a liquid adsorbent is used forseparation.

After the separation, the individual components, including biologicalagents, are detected, usually using the differences in the components'heat transfer properties. Although chromatographical methods are quitesensitive (from the low parts per million (ppm) range for gaschromatography to the moderate ppm range for liquid chromatography),they have serious disadvantages. The response times for these methodsare long and portability is very limited because it is difficult tominiaturize the equipment.

(b) Gel Electrophoresis. According to this method, particles of a gel(disperse phase) move in the dispersion medium under the influence of anelectric field. The detection of an agent, including a biological and/orchemical agent, is based on the principle that the speed of theparticles is proportional to the intensity of the electric field. Theelectric field causes the change of the electric potential on the borderbetween the gel phase and the dispersion medium and this change isrelated to the nature of the gel particles.

Although the gel electrophoresis-based concentrator can be madeportable, the method has a low sensitivity and very long response timesleading to only limited use of the method for detection of biologicaland/or chemical agents.

(c) Optical Absorption and Raman Spectroscopy. The method of opticalabsorption is based on the transition of electrons in an excited stateas a result of absorption of energy by the analyzed molecules inultraviolet, visible and near infrared areas of the spectrum (betweenabout 120 and 1,000 nanometers).

After such energy has been absorbed, wide-band spectra are obtained andthe analyzed molecule is identified.

In Raman spectroscopy, the analyzed substance is irradiated withmonochromatic radiation. When a beam of photons strikes the molecule,some photons undergo inelastic or Raman scattering and such Ramanscattered photons have different frequencies and produce a spectrum offrequencies in the scattered beam that constitute the Raman spectrum ofthe analyzed molecule. This spectrum is used for the identification ofthe sample.

Even though both the optical absorption method and the Ramanspectroscopy method provide a fast response time, sensitivity of both islow, and portability is limited. In addition, in the optical absorptionmethod, multiple compounds absorb the energy, thus producing amultiplicity of the resulting spectra and obfuscation of the moleculebeing sought.

(d) Fluorescence Spectroscopy (Tags). According to this method, visibleor ultraviolet radiation (at wavelengths between about 200 and 700nanometers) is first absorbed by the analyzed specimen. Thereafter, theexcited molecules return to the normal condition. This process isaccompanied by the emission of radiation. The emission spectrum of theradiation is thus obtained and used to identify the molecule.

The fluorescence spectroscopy method has fast response time and isamenable to designing a portable instrument. However, the method hasonly moderate sensitivity due to background noise contamination.

(e) Upconverting Phosphor. This method provides good backgroundrejection and has fast response times. Making a portable instrument forthis method is also possible. According to this method, uniformsubmicron microspheres of upconverting phosphors (UCPs) are synthesizedand coated with biologically active probes, such as antibodies. UCPs arematerials that emit visible light upon excitation with near-infraredlight. Functionalized UCP particles are used to selectively bind tocaptured target antigens. Visible emission following exposure to IRradiation indicates a target.

However, this method must be interfaced with pre-processing techniques,which limits the applicability and usefulness of UCPs.

(f) Mass Spectroscopy. The method of mass spectroscopy is based on theionization of a molecule, typically by bombarding the molecule withelectrons having energy between about 50 and 70 electron-volts. Theprocess takes place in a high vacuum environment (at least 10⁻⁶ torrvacuum is required). The ions created as a result are accelerated in anelectric field and separated in a magnetic field according to theirmass. A mass spectrum is then created showing the mass-to-charge ratiosfor various ions created as a result of the fragmentation of themolecule due to the bombardment and the relative amounts of such ions.This spectrum is used to identify the original molecule.

The sensitivity of this method is high, the response time is fast andminiaturization of the instrument and portability is possible. However,the method requires expensive and fragile equipment necessary to createand maintain a very high vacuum. The method requires high powerconsumption and the equipment is susceptible to clutter problems.

(g) Ion Mobility Spectrometry. This method is highly sensitive, has fastresponse time and the instrument can be portable. Ion MobilitySpectrometry (IMS) involves the ionization of molecules and theirsubsequent temporal drift through an electric field. Analysis andcharacterization are based on separations resulting from ionicmobilities rather than ionic masses; this difference distinguishes IMSfrom mass spectroscopy. However, serious problems of cross-coupling ofthe signal with temperature and pressure limit its applicability fordetection of biological and/or chemical agents.

As far as the instruments are concerned, there exist a number of currentgeneration sensors used in detection of biological and/or chemicalagents. Many of these sensors are constructed with a sensor transducerin combination with a biologically active surface.

A key element of these point detection systems involves theimmobilization of the biological and/or chemical agent. This issue isvery important for binding, isolating and concentrating the biologicaland/or chemical agent on the transducer as well as for maintaining theagent's structure, activity, and stability. Biological species ofinterest include various toxins, viruses, rickettsia, fungi, parasites,bacteria, or uniquely identifiable components or byproducts (DNA, RNA,proteins, sugars, etc.).

Typically, the surfaces on which biological agents are immobilized,comprise antibodies of a particular species of interest for trapping theparticular biological agent (antigen). These surfaces are integratedinto a transducer for sensing. Examples of such devices comprise surfaceacoustic wave (SAW) resonators, cantilever resonators, surface plasmonresonance reflectors, and fiber-optic based sensors that are coated withimmobilizing surfaces.

The antibody-antigen (biological molecule) binding is irreversible,which both benefits these detection methods and causes complications.The limitations of irreversible binding comprise baseline drift,saturation, and contamination of the sensor surfaces. Current generationpoint detection systems are often dependent on time-consuming molecularamplification processes that are needed to increase the signature of thepathogen or agent.

There is a need to have compact, low cost remote concentrators ofbiological and/or chemical species which:

-   (a) are very sensitive, compact in size, and able to detect very    small quantities of the compound in question;-   (b) have fast response times;-   (c) are portable and compact; and-   (d) are more reliable and have lower false alarm rates than    currently available sensors.

There exists no known prior art for compact concentrators satisfying allthese enhanced requirements. Yet the need for such is acute. For theforegoing reasons, there is a necessity for a compact low-cost sensorfor detection of very low amounts of biological and/or chemicalsubstances. The present invention discloses such concentrators which canserve to bolster the performance of any existing detection method byconcentrating the bio- or chemical-agent before analysis.

II. SUMMARY

This invention discloses a new class of solid-state concentrators ofbiological and/or chemical agents made from engineered, superlatticestructures, preferably based on group III-group V or group IV-group IVsemiconducting materials. The surface binding properties of suchsuperlattice structures can be controlled and manipulated, preferably,by optical illumination, or by other means, such as, for instance,electrical biasing.

The surfaces of the superlattice devices (edges of the structures) aredesigned to selectively complement the charge distributions(structural-chemical features) of a specific biological and/or chemicalmolecule.

The present invention takes advantage of the lengthwise similaritybetween the thickness of a superlattice layer and the typical distancebetween bonding sites of many biological and/or chemical molecules, aswell as between the overall thickness of the superlattice structure andtypical overall lengths of such biological and/or chemical molecules.The lengths of the biological molecules are typically within a rangebetween about 1 and 5 micrometers.

Indeed, the selective binding of the biological and/or chemical agentsby the method disclosed in this invention relies significantly on thesimilarity in length scale between biological and/or chemical agents orbiological molecule's (DNA, RNA, proteins, and the like) and engineeredsuperlattice structures. In both cases, the fine resolution chargedistributions lie in the 3-10 Angstroms scale and the coarse lengths arein the 2-10 micrometers range. For example, the dimensions between basepairs that serve to bind the individual strands of DNA are on the orderof 5 Angstroms.

Similarly, engineered superlattice structures can be fabricated withisolated layers on this length scale.

Illumination of the devices is preferably used to control and activatecarriers (electrons and holes) in specific layers in the superlatticestack. The illumination results in a unique charge distribution on theedge of the stack.

Superlattice structures are designed in such a way that theabove-mentioned illumination-induced surface charge distributioncontrols the binding properties for a particular biological and/orchemical molecule.

The optical control of the surface binding-properties allows the devicesto quickly, selectively, and reversibly trap and release biologicaland/or chemical agents. The selective binding and detachment forms thebasis of the enhanced detection scheme which is a subject matter of thepresent invention.

According to a first aspect of the invention, a concentrator ofbiological or/and chemical materials is provided, the concentratorcomprising an engineered superlattice structure, the structurefabricated out of semiconducting materials, preferably group III-group Vsemiconducting materials, or group IV-group IV semiconducting materials.Preferably, the superlattice structure comprises alternating layers ofelemental group and binary group or ternary group semiconductingmaterials; or alternating layers of binary group and binary group orternary group materials. For example, in a superlattice structurecomprising alternating layers of elemental group and binary group orternary materials, the elemental group materials comprise group IVsemiconducting materials. If, for example, the superlattice structurecomprises alternating layers of binary and ternary group materials, thebinary semiconducting materials may comprise gallium-arsenide,gallium-phosphide, silicon-germanium, silicon-carbon,gallium-antimonide, indium-arsenide, indium phosphide,indium-antimonide, aluminum-arsenide, or aluminum-antimonide, and theternary semiconducting materials may comprise aluminum-gallium-arsenide,silicon-germanium-carbon, indium-gallium-phosphide,indium-gallium-arsenide, indium-aluminum-arsenide,aluminum-gallium-arsenide, or aluminum-gallium-antimonide. Those skilledin the art will understand that the materials other than those listedabove may comprise the binary group or ternary group materials that maybe used. In an alternative embodiment, the superlattice structure maycomprise alternating layers of silicon and silicon-germanium, siliconand silicon-carbon, silicon and silicon-germanium-carbon, orsilicon-germanium and silicon-carbon semiconducting materials.

According to a second aspect of the invention, a method for detectingbiological and/or chemical materials comprises steps of providing aconcentrator of biological and/or chemical materials comprising anengineered superlattice structure fabricated out of semiconductingmaterials, exposing the concentrator to the biological and/or chemicalmaterials, trapping and concentrating the biological and/or chemicalmaterials on a surface of the superlattice structure by activating thesuperlattice structure, detecting the biological and/or chemicalmaterials, and releasing the biological and/or chemical materials fromthe superlattice structure.

III. BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention will become betterunderstood with regard to the following description, appended claims,and accompanying drawings.

FIG. 1 is a schematic diagram showing a cross-sectional view of apreferred multi-layer structure designed to capture specific biologicaland/or chemical molecules.

FIG. 1 a is a schematic diagram showing a perspective view of apreferred multi-layer structure activated by the method of electricalbiasing.

FIG. 2 is a schematic diagram showing a structure of a cell membranewith a lipid bi-layer.

FIG. 3 is a schematic diagram illustrating the similarity of the lengthscale and selective binding between an individual strand of DNA and anengineered superlattice structure.

FIG. 4 is a schematic diagram showing the energy gap for GaAs.

IV. DETAILED DESCRIPTION

1. The Invention in General.

The concentrator of this invention preferably comprises a multi-layerengineered superlattice having alternating layers made of preferablybinary group III-group V semiconducting materials and of preferablyternary group III-group V semiconducting materials. FIG. 1 shows across-section of a gallium-arsenide (GaAs)/aluminum-gallium-arsenide(Al_(x)Ga_(1-x)As) multi-layer structure which is a preferredsuperlattice, concentrator structure designed to capture a specificbiological and/or chemical molecule. In Al_(x)Ga_(1-x)As, x is a molefraction of Al in aluminum-gallium-arsenide, x preferably being within arange of between about 0.16 and about 0.8. (For AlGaAs x is about 0.16).

As shown on FIG. 1, the superlattice structure 100 comprises alternatinglayers 1 of GaAs and 2 of Al_(x)Ga_(1-x)As. The structure 100 ispreferably grown by molecular beam epitaxy (MBE). The layers 1, 2 may befabricated on a substrate 30. MBE methods are known to those reasonablyskilled in the art of making semiconductor devices.

The superlattice structure usually comprises between about 50 and about1,000 alternating layers 1 and 2, each layer typically having athickness of between about 3 and about 10 Angstroms. The total thicknessof the entire superlattice 100 is typically between about 1 and about 5micrometers.

The preferred geometry of the superlattice structure 100 is such thatthe edges 101 of the superlattice are exposed to the contaminatedenvironment. In order to achieve this goal, standard etching methodsknown to those skilled in the art are employed, including wet-chemicaletching and reactive ion etching techniques. Arrays of mesa or gradingtype structures are preferred structures allowing significant surfaceareas of the edges 101 to be accessed by the contaminated environment.

As will be discussed below, the superlattice structure is used forselective binding and detecting of phosphoglyderides, DNA and RNAstrands. For simple organisms, such as bacteria or bacteriophages, thenumber of nucleotides in their DNA strands is typically within a rangebetween tens and hundreds of thousands, and certainly less than twomillion.

The superlattice required for detection of such simple organisms iscapable of being manufactured by the MBE method. The MBE is a preferredmethod of manufacturing of the superlattice. In addition, a MetalOrganic Chemical Vapor Deposition (MOCVD) method, known to those skilledin the art, can be also used as another preferred method of manufacture.

In addition to the preferred GaAs/Al_(x)Ga_(1-x)As structure, othergroup III-group V-based superlattice systems can be used, as long as thematerials of such alterative systems can sustain a surface charge; thatis, these materials should not exhibit surface depletion effects.

The way the superlattice is used to reversibly bind and detectparticular biological and/or chemical agents will be discussed indetail, infra, by way of the following Examples. Examples 1 and 2 followdirectly. It should be mentioned that Examples 1 and 2 are given onlyfor illustrative purposes and should not be construed in any limitingway. In fact, those reasonably skilled in the art will understand thatthe method can be used for detecting other biological and/or chemicalagents as necessary.

2. Preferred Biological Molecules Detectable by the Method of theInvention.

The method and the superlattice of this invention are suitable fordetection of various biological materials, preferably, strands of DNAand RNA and phosphoglycerides, as described below in Examples 1 and 2.

EXAMPLE 1 Selective Binding of Strands of Deoxyribonucleic andRibonucleic Acids (DNA and RNA).

FIG. 3 illustrates the similarity of the length scale and selectivebinding between an individual DNA strand and an engineered superlatticestructure 100. The DNA strand 300 comprises a sugar-phosphate backbone 7and a base 8. As seen from FIG. 3, nucleotide 9 of the DNA strandattaches itself to the superlattice layers 1 and 2 at the edge 101 ofengineered superlattice structure 100. Hydrogen bonds 10 are utilized inorder to achieve such attachment.

Optical addressing, e.g., illuminating of the superlattice stack 100, tobe discussed below, is used to activate specific layers that result in aunique surface charge distribution that complements the binding sites ofa particular biomolecule. Binding of the biomolecule to the surface, asmentioned above, utilizes low energy hydrogen bonds 10.

The fine resolution charge distributions lie in the 3-10 Angstroms scaleand the coarse lengths are in the 2-10 micrometers range. For example,the dimensions between base pairs that serve to bind the individualstrands of the DNA are on the order of 5 Angstroms.

In implementing the superlattice structure for detection of nucleotidesequences, complementary charge sequences are generated in thesuperlattice structure by one of a number of the methods, such aselectrical biasing or, preferably, by illuminating the surface of thesuperlattice, as described below. Such charge sequences match theoverall DNA or RNA molecule.

The fine features of the charge distributions are generated to bind eachof the bases found in DNA, such as, e.g., adenine (A), thymine (T),guanine (G), and cytosine (C). Each of these bases possesses a uniquebonding or charge arrangement. The complementary charge sequences on thesuperlattice are generated to mimic the charge distributions as found innature. These naturally occurring distributions are such that the chargedistribution of adenine complements that of thymine, and that ofcytosine-guanine.

EXAMPLE 2 Phosphoglycerides as Keys to Detection of Cell Membranes

Cell membranes contain distinguishing features enabling selectivebinding and general organism identification. They make up as much as 80%of the mass of their respective cells and serve as aqueous separationbarriers as well as structural bases to which certain kinds of enzymesand transport systems are bound. Most cell membranes are known tocontain 40% lipids and 60% proteins. A basic structure of a cellmembrane is schematically depicted on FIG. 2.

The membrane 200 shown on FIG. 2 comprises a fluid phospholipid bilayer3 (comprising phospholipid monolayers 3 a and 3 b) of mixed polarlipids, with their hydrocarbon-chain tails 4 oriented inwardly to form acontinuous hydrocarbon phase and their hydrophilic, polar heads 5oriented outwardly. In this bilayer 3 individual lipid molecules canmove laterally, giving the bilayer 3 fluidity, flexibility and acharacteristically high electrical resistance. Cell membrane 200 bindsproteins 6.

As is clear from the above discussion, phosphoglycerides, as a largeclass of complex lipids, form a major component of cell membranes.Therefore, the detection of phosphoglycerides is a key to detecting thepresence of cell membranes, leading in turn to the identification of abiological and/or chemical molecule. As will be subsequently discussed,it is the selective binding of the polar head groups 5 to theconcentrator surface of this invention, that forms the basis of thisembodiment of the present invention.

As mentioned above, phosphoglycerides possess a polar head group 5 and ahydrophobic, non-polar tail 4. Various kinds of phosphoglycerides differin their respective sizes, shapes and electric charges of the polar headgroups 5, due to their differences in functional groups. As a result,charge distributions, both spatially-wise and intensity-wise, aredifferent for polar head groups 5 of each phosphoglyceride, enablingselective binding of particular phosphoglycerides to speciallyengineered surfaces 101 with particularly tailored charge distributions.The most abundant phosphoglycerides found in higher plants and animalsare phosphatidylethanolamine (PEA, a polar head of which is shown informula I below), and phosphatidylcholine (PC, a polar head of which isshown formula II below). PEA and PC are the major components of mostanimal cell membranes.

For bacteria, the most important phosphoglycerides are cardiolipin, alsoknown as diphosphatidylglycerol (CL, a polar head of which is shown informula III below), phosphatidylglycerol (PDG, not shown), and0-aminoacylphosphatidylglycerol (APDG, not shown)

For all the phosphoglycerides discussed above, the open bonds onhydroxyls of the phosphoric residues lead to the hydrophobic tailportions of the particular phosphoglyceride and are connected to theposition “3” of 1,2-diacylglycerol.

The charge characteristics of the heads of the phosphoglycerides areknown. Typically, there are two to three fully charged sites, eitherpossessing an extra electron (negatively charged nucleophilic sites) orone-electron deficient (positively charged electrophilic sites), ordipole sites having both. The nucleophilic sites are located on oxygenatoms as shown in formulae (I), (II), and (III). The electrophilicsites, in cases of PEA and PC, are located on the quasi-quaternaryammonium-type atom of nitrogen.

The method of this invention utilizes the fact of the length-scalesimilarity between the thicknesses of the superlattice layers and thetypical distance between bonding sites on the polar heads of individualphosphoglyceride molecules (Angstrom scale) as well as distances betweenneighboring lipid molecules (nanometer scale).

For instance, the charged sites mentioned above are uniquely spatiallyseparated and the distance between the charged sites in CL, PEA, and PCwas previously measured by prior researchers to be about 7.6 Angstromsfor CL (formula III), about 6.0 Angstroms for PEA (formula I), and about4.9 Angstroms for PC (formula II). In addition to the charged sitesdiscussed above, many phosphoglycerides heads also possess polarizedsites due to the presence of chemical groups with a permanent dipolemoment, for instance, hydroxyl groups —OH or carbonyl groups C═O.

It should be borne in mind that, as is clear from the precedingdiscussion, all of the above-mentioned charged sites and polarizeddipole structures are parts of the lipid molecules forming cellmembranes. Hence, these charged fragments and dipole fragments attractand orient the lipid molecules towards the engineered surface 101 of thesuperlattice structure of this invention and bind the lipid molecules tothis superlattice's surface.

Of those charged and dipole fragments mentioned above, positivelycharged sites are attracted to the electron-rich areas of the activatedsurface of the superlattice structure of this invention, and negativelycharged sites—to the electron-deprived areas. Thus, thephosphoglycerides are bound to the surface and retained there, followedby the detection to be discussed subsequently.

The binding of the phosphoglyceride molecules to the surface can beillustrated as shown on FIG. 3 for the DNA strand, except that thebonding in case of phosphoglycerides occurs due to the attraction of thecharged sites to the electron-rich areas of the activated surface of thesuperlattice structure of this invention, as mentioned above, ratherthan due to the formation of hydrogen bonds, as occurs in case of theDNA strands. The phosphoglyceride molecules are attracted to, and bondto the GaAs portion of the superlattice.

3. Detection.

Using the above-described superlattice structure 100 for determinationof the presence and the amount of biological and/or chemical moleculesof a particular kind includes the steps of attracting the molecules bythe superlattice structure, concentrating of the molecules on thesurface of the superlattice, the detection of the molecules, and theirrelease. It should be understood, that the sensitivity of the methoddepends solely on the sensitivity of detection method chosen. Even if avery minimal amount (as low as a few parts per trillion) of thebiological and/or chemical materials is trapped by the superlattice, itstill can be detected, using suitably sensitive detection techniques.

In general, the concentrator is first exposed to the environmentcontaining the biological and/or chemical material being detected.Usually, such environment is a liquid, for instance, a solutioncontaining the biological and/or chemical material; however, detectionin a gas phase, for example, in the air, is also possible.

Next, the individual layers in the superlattice structure are preferablyactivated by shining particular wavelengths of light (shown as “hv” onFIG. 1) onto the superlattice structure 100, to produce a complex chargedistribution on its surface. Besides the activation of the individuallayers by optical means, any other alternative method of activation canbe used. Such methods of activation are known to those skilled in theart. An example of such an alternative method is the activation of theindividual layers by electrical biasing, as subsequently discussed.

The charge distribution in case of optical activation of the individuallayers will depend on the optical absorption properties of theindividual layers of the superlattice 100 and can be modified by varyingthe thicknesses and compositions of those layers, according to a chosendesign.

It should be understood that in order to trap a particular kind of abiological and/or chemical molecule, a surface charge distribution thatis specifically suited for this particular kind of the biological and/orchemical molecule is required. In other words, the surface chargedistribution produced by activating the individual layers in thesuperlattice must be specifically tailored to a particular biologicaland/or chemical molecule that is to be detected.

Stacking the layers with pre-selected absorption characteristicscombined with the specific doping of the layers, is used to produce suchspecific complex charge distributions. As a result of producing suchcharge distributions, particular biological and/or chemical moleculesare attracted to the surface of the superlattice and are trappedthereupon as described in detail hereinabove.

The period of the exposure of the superlattice structure to light isbetween a few seconds to a few minutes, depending, inter alia, on thekind of the biological and/or chemical molecule being detected, and onits concentration.

Generally, the time of the exposure should be such as to permit theamount of the biological and/or chemical substance sufficient for thedetection to be accumulated on the surface of the superlattice structure100.

In the case of the phosphoglycerides and the DNA/RNA strands, the timeof exposure will be between about 1 second and a few minutes, dependingon the concentration of the phosphoglycerides or the DNA/RNA strands inthe environment.

Describing particular details of the optical activation of the surfaceof the superlattice structure 100, it should be borne in mind that inorder to achieve such activation, the energy gap (the difference betweenthe valence band potential and the conductance band potential) must beovercome. From this basic requirement it follows, that the opticallygenerated free carriers in the superlattice structure are captured bythe GaAs layers and not by the Al_(x)Ga_(1-x)As layers due to thesmaller energy gap of GaAs relative to Al_(x)Ga_(1-x)As. Therefore, itis the GaAs layer that must be excited, and not the Al_(x)Ga_(1-x)Aslayer, because the latter layer just serves as an insulator of the GaAslayer and does not itself trap any molecules.

FIG. 4 shows the energy gap for GaAs, conduction, and valence band edgesas a function of position in the direction y (parallel to the layers ofan n-type GaAs/Al_(x)Ga_(1-x)As multi-layer structure near an exposedsurface). As a result of illumination of the structure, an electricfield is formed, which sweeps optically generated holes and electrons,respectively, towards and away from the surface.

A steady-state positive (hole) charge is maintained at the surface underconstant illumination. Similarly, a p-type GaAs/Al_(x)Ga_(1-x)Asmultilayer structure will yield a steady-state negative (electron)charge at the surface under illumination. As seen from FIG. 4, at roomtemperature, the energy gap for GaAs is 1.424 eV and is the differencebetween the energy levels for the conduction band 12 and the valenceband 11.

The calculations show that at room temperature the energy gap E_(g) forAl_(x)Ga_(1-x)As is computed according to equation (1)E _(g)=1.424+1.247x,  (1)wherein x is below 0.45.

Thus, for x=0.16 (the lowest preferred value for x as discussed above)E_(g) equals 1.624 eV.

For x above 0.45, the energy gap is computed according to equation (2):E _(g)1.900+0.125x+0.143x ²  (2)

Thus, for x=0.8, E_(g) equals 2.092 eV. The optical absorption edge inthe structure can be altered by varying the GaAs layer width (due toquantum confinement effects), or by changing its composition toAl_(y)Ga_(1-y)As, where y<x.

In view of the preceding discussion it is clear that for the preferredsuperlattice structure of this invention, the energy gaps andcorresponding wavelengths of the source of illumination are as shown inTable 1 below.

Table 1

Energy gap levels and corresponding wavelengths of the source ofillumination for the layers of the preferred embodiment of thesuperlattice of this invention

TABLE 1 Energy gap levels and corresponding wavelengths of the source ofillumination for the layers of the preferred embodiment of thesuperlatice of this invention Layer Energy gap, E_(g) eV Wavelength, nm.GaAs 1.424 870 Al, Ga, As between 1.673 (x = 0.2) between 741 (x = 0.2)and 2.092 (x = 0.8) and 593 (x = 0.8)

In view of the foregoing, the preferred range of wavelengths used toactivate the superlattice structure 100 is within a range of above about741 nm and below about 870 nm, more preferably, below about 800 nm, andmost preferably, about 750 nm. Illumination at wavelengths shorter thanthe lower limit of this range carries too much energy and will activatethe Al_(x)Ga_(1-x)As layer which activation should be avoided.Conversely, illumination at wavelengths longer than the upper limit ofthis range does not carry enough energy to activate the GaAs layer.Without such activation the device will not work.

As an alternative, non-preferred method of activating the individuallayers, electrical biasing is used, as shown on FIG. 1 a. For thispurpose, electrical current with voltage within a preferred range ofbetween about 3 Volts and about 5 Volts is used. For illustrativepurposes FIG. 1 a shows that the voltage is applied only to one GaAslayer 1, but it should be understood that the voltage is to applied toevery GaAs layer 1.

After the biological and/or chemical molecules have been trapped andconcentrated on the surface of the superlattice, they are detected byone of existing analytical methods known to those skilled in the art,preferably by the method of fluorescence spectroscopy, or lesspreferably by the method of upconverting phosphor. In the alternative,the superlattice structure 100 can be incorporated into other devicessuch as surface acoustic wave (SAW) resonators, cantilever resonators orsurface plasmon reflectors. The selective and switchable binding of thesuperlattice structures controlled by optical modulation (or use ofanother means), enables these devices to effectively detect the presenceof particular bio-agents.

Finally, after the biological and/or chemical molecule was trapped andidentified, it is released from the superlattice sidewall, for instance,by illumination with a different wavelength of light, or wavelength withan energy greater than the bandgap of AlGaAs spacer layers. Generally,the wavelength used for this purpose is shorter than about 500nanometers. Following such detachment of the biological and/or chemicalagent, the device is ready to be used again.

Having described the invention in connection with several embodimentsthereof, modification will now suggest itself to those skilled in theart. As such, the invention is not to be limited to the describedembodiments except as required by the appended claims.

1. A concentrater of biological and/or chemical materials comprising anengineered superlattice structure, wherein said structure is fabricatedout of semiconducting materials, said structure having a surface withsurface binding properties for binding and releasing said biologicaland/or chemical materials.
 2. The concentrator as claimed in claim 1,wherein said surface binding properties are controlled by opticalillumination or electrical biasing.
 3. The concentrator as claimed inclaim 1, wherein said semiconducting materials comprise group III-groupV semiconducting materials, or group IV-group IV semiconductingmaterials.
 4. The concentrator as claimed in claim 1, wherein saidbiological and/or chemical materials comprise phosphoglycerides,deoxyribonucleic acid, or ribonucleic acid.
 5. The concentrator asclaimed in claim 1, wherein said superlattice structure comprises aplurality of alternating layers made of elemental semiconductingmaterials and binary or ternary semiconducting materials of saidsemiconducting materials or wherein said superlattice structurecomprises a plurality of alternating layers made of binarysemiconducting materials and binary or ternary semiconducting materialsof said semiconducting materials.
 6. The concentrator as claimed inclaim 5, wherein said layers made of elemental semiconducting materialscomprise a material selected from group IV materials.
 7. Theconcentrator as claimed in claim 5, wherein said superlattice structurecomprises alternating layers of binary and ternary semiconductingmaterials and said binary semiconducting materials are selected from thegroup consisting of gallium-arsenide, gallium-phosphide,silicon-germanium, silicon-carbon, gallium-antimonide, iridium-arsenide,indium-phosphide, indium-antimonide, aluminum-arsenide, andaluminum-antimonide, and said ternary semiconducting materials areselected from the group consisting of aluminum-gallium-arsenide,silicon-germanium-carbon, indium-gallium-phosphide,indium-gallium-arsenide, indium-aluminum-arsenide,aluminum-gallium-arsenide, and aluminum-gallium-antimonide.
 8. Theconcentrator as claimed in claim 3, wherein said superlattice structurecomprises a plurality of alternating layers of silicon andsilicon-germanium, silicon and silicon-carbon, silicon andsilicon-germanium-carbon, or silicon-germanium and silicon-carbonsemiconducting materials.
 9. The concentrator as claimed in claim 5,wherein said superlattice structure has a thickness within a range ofbetween about 1 micrometer and about 5 micrometers.
 10. The concentratoras claimed in claim 5, wherein each layer of said alternating layers hasa thickness within a range of between about 3 Angstroms and about 10Angstroms.
 11. The concentrator as claimed in claim 5, wherein saidstructure is fabricated by a method of molecular beam epitaxy (MBE). 12.The concentrator as claimed in claim 5, wherein said surface has edges,said edges having a mesa structure or a graded structure.
 13. Theconcentrator as claimed in claim 12, wherein said edges are formed by amethod of wet-chemical etching or by a method of reactive ion etching.